In the field of oil and gas exploration and extraction, pressure sensors are customarily used at the surface for reading data generated by a pulse generator (or a pulser) located downhole. The data travels through the drilling mud along the wellbore, typically in the form of short pulses providing a binary encoded signal, to the surface. Some of the telemetry schemes used for transmitting data from near the drill tool in a wellbore to the surface include Pulse Position Modulation (PPM) and Pulse Width Modulation (PWM). These modulation techniques rely on sending a sequence of acoustic pulses encoding data to be telemetrically transmitted to the surface through the drilling mud in a mud flow. In a PPM scheme, the position of a pulse in a given time slot within a selected packet of time slots indicates a value for a symbol. Some configurations include a differential PPM (DPPM) scheme, in which the location of a current pulse is determined in relation to the previous pulse, rather than within a specified time window. In a PWM scheme, the length of a sequence of consecutive pulses within the packet is correlated to a value for the symbol. The closer in time that pulses can be placed with respect to each other, the more data can be sent in the same amount of time. This is especially desirable when the amount of data transmitted to the surface is exceedingly large (e.g., image data files).
In some instances, this data can be distorted and attenuated during this process. For example, acoustic pulses can be distorted due to dispersion and attenuation effects as they travel along the wellbore. Acoustic pulses also can be reflected at various points of the mud flow system and create one or more echoes or reflections in a pulse sequence. Pulse reflections may occur, for example, at the surface pumps in a drilling system, or at any bend in the plumbing associated with a mud flow, in the drilling system. More generally, a plurality of acoustic pulses originating from the same signal pulse at a source may follow multiple paths and arrive at a sensor at slightly different time, thereby interfering with other ‘true’ signal pulses arriving at the sensor. Spurious ‘echo’ and multi-path interference effects and possibly others can negatively impact the quality of the information content of the pulse sequence, increasing Bit-Error-Rate (BER), as the signal-to-noise ratio (SNR) is reduced. For example, a reflected pulse may overlap with a subsequent signal pulse, distorting the transmitted message in a phenomenon known as inter-symbol-interference (ISI). Therefore, a minimum time is typically set between pulses so that the pulse reflection does not affect the following pulse. This minimum pulse time (MPT) determines the maximum data rate that can be transmitted to the surface in a mud pulse telemetry application.
Attempts to resolve the pulse ‘echo’ problem include increasing the time lapse between successive pulses in the signal to identify pulse ‘echoes’ from a widely spread pulse sequence, or letting the pulse echoes die off before the next signal pulse arrives. Other approaches include a “training pulse sequence” transmitted at pre-selected times. A training pulse sequence is a pre-selected sequence of pulses known to the transmitting party and to the receiving party. Knowledge of the ideal pulse sequence and comparison with the received pulse sequence enables a data processor to perform the appropriate adjustments to received signals. However, utilizing training pulse sequences as means to reduce BER may not be reasonable, as it has an undesirable time cost associated with it since real operations have to be off-line while the training pulse is run. Some of the above approaches limit the number of pulses that can be placed on a given time interval, thereby introducing a lower limit to the time length of a data frame and an upper limit to the data transmission rate. This compromise is undesirable in conditions where large amounts of data are transmitted in real-time logging while drilling (LWD) applications.